Roller cone bits, variously referred to as rock bits or drill bits, are used in earth drilling applications. Typically, they are used in petroleum or mining operations where the cost of drilling is significantly affected by the rate that the drill bits penetrate the various types of subterranean formations. That rate is referred to as rate of penetration (“ROP”), and is typically measured in feet per hour. There is a continual effort to optimize the design of drill bits to more rapidly drill specific formations so as to reduce these drilling costs.
Roller cone bits are characterized by having roller cones rotatably mounted on legs of a bit body. Each roller cone has an arrangement of cutting elements attached to or formed integrally with the roller cone. A roller cone bit having two cones was invented in 1908 and is the predecessor of the more common three-cone bit. Two-cone drill bits greatly improved drilling rates in the early 1900's, but were found to suffer severe near bit vibrations, which resulted in extensive damage to downhole tools. Three-cone bits gradually replaced two-cone drill bits because of an increase in stability and reduction in vibrations during drilling. Historically, the advantage maintained by two-cone drill bits is that they are generally able to drill faster than three-cone bits. Additionally, for drilling small holes, using three-cone bits, as opposed to two-cone bits, requires smaller legs that will be subjected to high loads through the roller cones, which are rotatably mounted. Two-cone drill bits are able to offer relatively larger legs for such hole sizes.
The two legs of most prior art two-cone drill bits are disposed substantially opposite of each other (i.e., 180 degrees apart) to evenly distribute the weight on bit (“WOB”) while drilling. However, recently it has been found that improvements to the stability of two-cone drill bits may be made through the orientation of roller cones and/or changes in cutting structure arrangements on the roller cones.